Suppose you have multiple entry files and each requires jquery. In this
case, each output file will contain jQuery, slowing down your user's experience.
To solve this, you can extract the common libraries to a "shared" entry file
that's included on every page.
Suppose you already have an entry called app that's included on every page.
Update your code to use createSharedEntry():
Before making this change, if both app.js and store.js require jquery,
then jquery would be packaged into both files, which is wasteful. By making
app.js your "shared" entry, any code required by app.js (like jQuery) will
no longer be packaged into any other files. The same is true for any CSS.
Because app.js contains all the common code that other entry files depend on,
its script (and link) tag must be on every page.
The app.js file works best when its contents are changed rarely
and you're using long-term caching. Why?
If app.js contains application code that frequently changes, then
(when using versioning), its filename hash will frequently change. This means
your users won't enjoy the benefits of long-term caching for this file (which
is generally quite large).